The following packages have unmet dependencies.
libwebkitgtk-1.0-0 : Depends: libgail18 (>= 1.18.0) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

and then running

sudo apt-get install libgail18

gives me this error

The following packages have unmet dependencies.
libgail18 : Depends: libgtk2.0-0 (= 2.24.10-0ubuntu6) but 2.24.10-1oneiric6~ppa is to be installed
E: Unable to correct problems, you have held broken packages.

The bit that caught my attention was but 2.24.10-1oneiric6~ppa is to be installed - but I don't know what to do with this. I've rebooted and the error messages are the same. Can anyone help?

Thanks for your reply - having done all of the above I've still got the same issue. Full output here : pastebin.com/PCMN0Min
–
MartynMay 9 '12 at 16:26

No prob, here's what you need to do. Install synaptic package manager and search for Gimp related files. Remove all Gimp relating files. Also check if you can run the repair for broken packages in synaptic.
–
Srinivas GowdaMay 9 '12 at 16:39

I've done this and have removed everything I can see which is explicitly gimp related but still getting the same error message. Here're the left over installed items when searching for gimp - tinypic.com/view.php?pic=2qxrp0o&s=6
–
MartynMay 10 '12 at 8:55

I found a solution!
I also had this problem, must have tried every command of apt-get, purging the repository, manually deleting all other source lists under /etc/apt/, updating, etc - but still the package manger was saying that it wanted to install the '2.24.10-1oneiric6~ppa' version even though I knew that there was no trace (that I knew of) of any ppa for apt-get.
I tried playing around with aptitude and that downgraded libgtk2.0-0 for me and everything started installing correctly. I don't remember exactly what I used since I really only use aptitude to search for packages in my konsole but searching for libgtk2 then marking it for upgrade did the trick. Incomplete directions below and ignore the quotes:

sudo apt-get install aptitude
sudo aptitude
press "/" to start searching
type in the name of the offending package
you may have to press "/" then enter again, do until you find it
press "+" to upgrade or "i" to install
This is when aptitude offered to downgrade
press "g" to do the upgrade/installation